1. Field of the Invention
This invention relates in general to fluid production systems and, in particular, to fluid production systems using tandem progressive cavity pumps.
2. Brief Description of Related Art
In some well completions, use of progressive cavity pumps, or progressing cavity pumps (PCPs), is preferred to produce well fluids from the completed well to the surface. The PCPs are suspended within a production zone on a string of tubing and operated to lift well fluid to the surface. PCPs may be preferred in part because they operate at lower speeds. Lower speed operation provides a costs savings due to the ability of the PCP to operate with standard equipment rather than heavily overbuilt equipment. Lower operating speeds also allow the PCPs to operate for longer periods of time without repairs or replacement. Still further, the lower operating speeds allow PCPs to handle well fluids with suspended solid matter better than other pumping systems. This is also a result of the PCP pumping mechanism which moves the fluid through the pump without flinging it against the pump stator. This decreases the stress on the pump during operation. In addition, it prevents damage to the pump caused by the impact of suspended solids on the pump housing that may cause pitting and eventual pump leakage.
Unfortunately, PCPs are unable to overcome as much head as other pump types, such as electric submersible pumps (ESPs). ESPs are typically centrifugal type pumps. Because of this, PCPs may not be used in well completions where the production zone is beyond 5,000 to 7,000 feet from the well surface. In those instances, other pump types capable of producing the well fluid to the surface, beyond the 5,000 to 7,000 feet range, must be used. This can lead to problems when the pumped fluid has a high suspended fluid content. While it is possible to use non-PCPs in wells having a high content of suspended solid matter in the well fluids, the pumps are likely to need repair and replacement at more frequent intervals. This is a result of the higher operating speeds and pumping mechanisms that may fling the suspended solids against the pump housing. More frequent repair and replacement increases the costs of production. As the time costs and production costs to continually repair or replace the downhole pump increase, the areas in which hydrocarbons may be feasibly developed are diminished due to decreased profitability margins for the well. Therefore, there is a need for a PCP that can lift well fluids beyond the standard 5,000 to 7,000 feet, thus avoiding use of ESPs.